Inductive proximity sensors are often designed based on an eddy current killed oscillator (ECKO) principle. In such proximity sensors, an inductor-capacitor (LC) oscillator is used to generate an alternating current (AC) magnetic field adjacent to an electrical coil of the inductor such that when a metallic target approaches the coil, eddy current is induced in the target. The induced eddy current generates an energy loss that can damp or collapse the LC oscillator. An electronic circuit is often employed to detect changes of oscillation amplitude and compare the detected changes to a predetermined threshold. If the oscillator amplitude drops below the threshold level, the output of the proximity sensor is updated to indicate the presence of a target in the vicinity of the proximity sensor.
Inductive proximity sensors operating based on the ECKO principle are shown to have a number of drawbacks. For example, due to their high thermal sensitivity, these sensors may suffer from having short sensing distances. In fact, these sensors may have much shorter sensing distances for non-ferrous metal target in comparison to ferrous targets. Additionally, since inductive proximity sensors are not immune to outside magnetic fields, external magnetic fields may saturate the ferrite core used in the sensing coil and cause malfunction of these sensors. Inductive proximity sensors may also suffer from limited switching speed.
In order to achieve equal sensing distance for ferrous and non-ferrous metals, Tigges (U.S. Pat. No. 5,264,733) employs a transmitting coil along with two receiving coils to detect field disturbances in place of eddy current loss. As the operating frequency increases above a certain limit, ferrous and non-ferrous targets tend to affect the AC magnetic field similarly. As such, at sufficiently high operating frequencies, equal sensing distances for both ferrous and non-ferrous targets can be achieved. Tigges also achieves immunity to external magnetic fields by employing three air cores. However, since Tigges relies on relative displacement, mechanical geometry, and number of turns in the coils to set sensing distance, the sensing distance is not electrically tunable and may be prone to temperature drift due to thermal deformation of its mechanical parts. Additionally, since at least three coils are required to implement the transmitting and receiving coils, the physical implementation may be expensive.
Another multi-coil design based on transformer coupling principle is described by Kühn (U.S. Pat. No. 7,463,020). Kühn arranges all coils as concentric circles on the surface of a printed circuit board (PCB) and employs one transmitting coil and at least two receiving coils, along with a PCB to implement the coils, while attempting to ensure that the coupling factors among coils are stable and repeatable. The receiving coils are positioned on the same plane and as such the sensitivity of the sensor to a target may be relatively lower compared to other known methods in the art.
Yet another method for achieving equal sensing distances for ferrous and non-ferrous targets is described by Tigges et al. (U.S. Pat. No. 4,879,531). Tigges et al. employs an oscillator that includes two LC tanks. The first LC tank determines oscillating frequency. The second tank includes a sensing coil and is used to amplify the oscillator. In order to achieve equal sensing distances for both ferrous and non-ferrous metals, the resonant frequency of the first tank and the critical impedance value of the second tank are tuned to the coordinates of the point of intersection of the impedance frequency characteristics of the second tank, which is affected by ferrous and non-ferrous target respectively. However, Tigges method requires a complicated tuning process to achieve equal sensing distances. Additionally, thermal sensitivity may limit achievable sensing distances and the addition of the two coils may increase the cost of implementation.
Tomioka et al. (U.S. Pat. No. 5,034,704) also employs two LC tanks to achieve equal sensing distances for ferrous and non-ferrous metals. In implementation, Tomioka employs an oscillator circuit and as such has similar advantages and disadvantages as Tigges.
Mounting an inductive proximity sensor to a metallic material may result in shifting the sensing distance of a sensor or, in more extreme cases, locking the output of the sensor, resulting in sensor malfunction. Various shield design techniques have been introduced and implemented in the literature. Such methods include placement of a metallic ring around the sensing coil (i.e., passive approach) or use of a compensating coil (i.e., active approach). While such methods help build shielded or embeddable inductive proximity sensors having standard sensing distances, for sensors with extended sensing distances, the effectiveness of the shield design remains a major issue.
Due to their outstanding resistance to severe application environments (e.g., dirt, wetness, existence of chemical liquids, etc.), inductive proximity sensors are widely used. On-line calibration methods or learning mechanisms exist to compensate for influence of mounting material. Unfortunately, placement of adjustment buttons on a sensor body may reduce the seal rating of the sensor. In order to resolve this issue, some sensor designs employ a control box that connects to the sensor using a so-called “pig tail” cable connection. The control box includes control modules that are used to program the sensor and compensate for the influence of the mounting metallic material. However, the control box may significantly increase product cost. Additionally, once a sensor is installed, the calibration process may require the sensor to be powered up. Further, sensors using the control box calibration method may be accidentally re-calibrated, resulting in failure of the sensors during operation or service.